The evolution of red blood cell shape in fishes

The size and shape of Red Blood Cells (RBC) provide key information on life history strategies in vertebrates. However, little is known about how RBC shape evolved in response to environmental factors and the role of trait evolutionary mode. Here, we analyzed RBC morphometrics in a set of Teleostei and Elasmobranchii species testing the hypothesis that phylogenetic relationship determines species occupation of morphospace. We collected data on cell and nucleus area, cell and nucleus volume, nucleus:cytoplasm ratio, and shape factor for 65 species belonging to 28 orders. Then, we built a phylomorphospace separately for sharks and bony fish. To test if phylogenetic relationships predicted phenotypic similarity, we calculated multivariate phylogenetic signal. We also estimated the evolutionary rate of RBC shape for each node and tip using ridge regression. Finally, we tested if water column position influenced RBC shape using a PGLS. Saltwater species were more clustered than freshwater ones in the phylomorphospace, suggesting clade disparity. Accordingly, the rate of evolution for shape was highly heterogeneous, with significant decrease in Acanthopterygii. Neither habitat nor body size influenced RBC shape. In conclusion, RBC shape seem to have evolved in response to multiple selective pressures independent of life history characters.